Thermal management of advanced space systems presents several problems, including the need for low mass, efficient heat rejection over a wide range of power loads, and the need for thermal control structures capable of heat removal from thermal loads of systems over wide temperature ranges.
Space radiators capable of high emissivity at moderately low operating temperatures coupled with low solar absorptivity, which are highly survivable and capable of withstanding both natural and hostile threat environments are therefore needed. Other considerations in such space radiators include a decreased payload mass, provision of high power conversion systems in space, extension of the operating temperature range of moderately low temperature radiators, and the provision of lightweight, highly survivable thermal management systems for the heat rejection radiator.
Current radiator designs for the power levels of interest use either pumped loops or heat pipe radiators. Pumped loop radiators are simpler and may be less massive, but for long duration missions, the inherent simplicity and redundancy of a heat pipe radiator, without moving parts, makes it more reliable, and is usually the design of choice for advanced systems. Near term space systems must reject approximately 1 kW to 10 kW of waste heat at temperatures of the order of 300 degrees K. to 400 degrees K. Advanced power systems will require radiators capable of rejecting hundreds of thousands of kilowatts at temperatures as high as approximately 1000 degrees K.
Because of the nature of their mission, these systems must also survive attacks from lasers, nuclear X-rays, and kinetic energy weapons, and withstand the natural space environment. Efficient radiators which can reject these thermal loads and survive these hostile threats are required in addition to the need for more effective radiators capable of long term operation in the natural space environment.
Efficient heat rejection requires that the radiators should have high infrared emissivity, and high thermal conductance throughout. For low temperature radiators, e.g., temperatures less than approximately 400 degrees K., low solar absorptivity is an important performance parameter. Additionally, the radiator must be highly reliable, have a low stowage volume for launch, withstand high g launch loads and maneuvers, and satisfy specific platform interface requirements. To survive the hostile threats, as well as the natural space environment, the radiator must be designed with carefully chosen combinations of materials. The laser threat demands that materials be able to withstand high temperatures and/or reduce the heat load by either reflecting, transmitting or absorbing and reradiating the incident energy. High laser reflectance is usually inconsistent with the requirement for high infrared emissivity, and heat pipe fluids effective at the temperatures of interest are not able to absorb the high laser fluxes expected for the strategic defense initiative systems. Therefore, the radiator design must somehow minimize the laser energy reaching both the working fluid and the thermal management system to which the radiator interfaces, and also keep the maximum temperature to which the heat pipe and fluid are exposed at an acceptable level.
The current technology for protecting the radiators from incident laser energy is in the form of rugate coatings on the radiator surfaces that are able to selectively reflect the laser radiation while maintaining high emissivity. However, these coatings are vulnerable to changes in laser wavelengths and are easily damaged and abraded by space dust and micrometeorites. Furthermore, the rugate coatings can not survive nuclear X-ray exposure at anticipated levels.
In general, nuclear X-rays pose a particularly stressing design requirement, since the X-ray absorption causes surface destruction by vaporization and high thermal stress shock loads transmitted through the exposed material. Low atomic number materials, also known as low Z materials, tend to absorb less X-ray energy. High atomic number, or high Z materials tend to absorb all of the nuclear X-rays within a thin layer of the material. Such total absorption initiates complex interactions leading to damage.
The kinetic energy weapon threat can either be a single large hypervelocity projectile of several grams, or a cloud of smaller hypervelocity debris, each particle having a mass of between 10 milligrams to 1 gram. For either threat it is impractical to armor the radiator to survive the impact. Instead, sufficient redundancy is included in the design to assure that the radiator can satisfy the heat rejection requirements after an attack. The space debris and micrometeoroid threat, however, includes very small particles, having a mass as low as 0.00001 gram with such large fluences that every pipe will suffer an impact during the lifetime of the platform. The radiator must be armored to survive these impacts and the emissivity of the surface maintained in spite of this constant surface abrasion, especially for long term missions.
Another consideration in the design of a radiator system is its vulnerability to the ambient environment consisting of atomic oxygen, and various forms of naturally occuring radiation. Materials for the radiator and any coatings need to be carefully chosen to avoid excessive performance degradation over periods of several years when exposed to the on orbit environment.